The present invention relates to a method for manufacturing integrated arrays of sensors, in particular chemical sensors or physical sensors e.g., field-effect chemical sensors. Such sensors are typically based on metal-insulator-semiconductor or metal-conductor structures. The present invention improves the quality of the top layer that serves as the chemically sensitive part of the device.
1. Introduction
It is known that catalytic metals can be used as gate electrodes for chemically sensitive field-effect devices such as transistors, capacitors, diodes, etc. These metal-insulator-semiconductor or metal-semiconductor devices may be used to measure small concentrations of molecules in the gas phase or ions in the liquid phase. The parameters that are normally changed when manufacturing field-effect sensors with different sensitivities towards different chemical compounds are the material and thickness of the top electrode. In this way the individual sensors in an array can be made sensitive to different compounds as well as differently sensitive to the same compound increasing the accuracy in the detection. There is an increasing need for arrays of chemical sensors with different chemical sensitivity patterns, e.g., for use in so-called electronic noses. Field-effect chemical sensors have the advantage that they can be manufactured using standard production methods for integrated electronic. devices. Thus, it is possible to manufacture small and cheap arrays of field effect sensors with different chemical sensitivities on the same chip simply by using different top layers. In order to minimize the production costs of the devices, the arrays should be made with as small separation as possible between individual sensors. It is also very important that the to player electrodes, which can have a thickness down to some nanometers, are deposited with a high lateral precision, purity, and uniformity to achieve a good reproducibility and high quality sensors.
2. Prior art
The deposition of the top-layer electrodes on field-effect chemical sensors is made either using standard lift-off techniques or using shadow-mask techniques. A lift-off procedure typically includes the following steps: deposition of the resist, masking, exposure, and developing of the resist to create openings in the resist, deposition of the sensor top-layer material, and cleaning and rinsing of the surface to remove the resist. This entire procedure has to be done once for each type of top-layer material in the array, meaning that the sensing surfaces will be subjected to a number of lift-off procedures. Each lift-off procedure is, however, associated with a risk of contaminating the to player electrode or its substrate with small amounts of remaining resist. The result is impaired adhering between the deposited top layer and the substrate as well as contaminated top layers. Since the top layer controls the specific sensitivities of the sensor this seriously impairs the sensors recognizing ability. One way of circumventing this is to use very strong resist removers, but this instead increases the risk that the top-layer electrode peels off.
By instead using shadow-mask techniques, the problems associated with lift-off are avoided since no resist layers have to be processed and removed. Instead a precision-machined mask of some suitable material is used to define the top layer or electrode areas instead of the patterned resist mask. However, the shadow-mask technique is much less accurate than lift-off procedures and thus requires an unnecessary large electrode area and separation between individual sensors. Furthermore, the shadow-mask technique requires a complicated manual positioning step for each type of top layer, making it more time-consuming and expensive than lift-off procedures. Also the edges of the applied areas will be less precisely shaped resulting in variations between the different sensors.
The object of the invention is to eliminate the above problems to obtain a fabrication method giving the desired quality, control and repeatability. In accordance with the present invention, there is provided a method for fabricating integrated sensor arrays on a common substrate, comprising a first step of developing a resist mask on the surface of the substrate, wherein the resist mask is provided with openings for all areas of the substrate where a deposition is to take place in subsequent steps. Then, a first shadow mask is located over the resist mask from the first step and a first deposition of materials takes place in which the first shadow mask screens and protects all openings in the resist mask that are not to be subject to deposition leaving only those where the deposition is to take place. Thereafter, a second shadow mask is located over the resist mask and a second deposition takes place in areas of the openings in the resist mask on the substrate that are not screened by said second deposition. Additional shadow masks then are used to expose the desired openings in the resist mask to a treatment, and, after all depositions have been carried out a different openings in the resist mask, the last shadow mask is removed and then the resist mask is removed. Further advantageous improvements of the method are described in the following description of a preferred embodiment described with reference to the enclosed drawings wherein like numerals depict like parts, and wherein: